1. Field of the Invention
The present invention relates to four-cycle engines and, in particular, small-sized engines integrated into portable machines and the like.
2. Description of the Related Art
Engines integrated into portable machines such as brush cutters and chain saws must be small in size and light in weight. The Japanese Utility Provisional Model Publication No. HEI-4-117103 describes a small-sized four-cycle engine suited to such portable machines. In four-cycle engines, the valves (an intake valve and an exhaust valve) must be opened once each time the crankshaft turns twice. A cam shaft, therefore, for opening and closing the valves is normally provided independent of a crankshaft. The cam shaft receives power from the crankshaft via gears (a crank gear and a cam gear), etc. and turns one half of the number of turns of the crankshaft according to the gear ratio of the gears or the like.
Valve actuating mechanisms such as push rods and rocker arms for valve opening and closing are connected to the cams on the cam shaft, and the intake valve and the exhaust valves are opened and closed at the above-mentioned frequencies. In the engine described in the above-mentioned gazette, a cam shaft is provided independent of the crankshaft.
When a cam shaft is provided for opening and closing valves, the number of parts of an engine will increase by the number of the cam shaft and the related gear, and the weight and size of the engine will increase accordingly. Keeping this point in view, the Japanese Patent Provisional Publication No. SHO-61-229906 proposes a four-cycle engine wherein no cam shaft is used to open or close the intake and exhaust valves. FIG. 4 of the instant application is a longitudinal sectional view of the engine disclosed in the gazette. A special guide portion 11' functioning as an equivalent of the above-mentioned cam is formed on a crankshaft 10' and one end of a valve actuating mechanism 40' is connected (contact engagement) with the surface of the guide portion 11'. The guide portion 11' is in the form of a groove having a route that returns to the starting point after two turns over the external circumference of the crankshaft 10'. A profile similar to a cam is made in the bottom of the groove, said profile having various radii relative to the center of rotation of the crankshaft (the profile curve gives one cycle when turned twice). When the crankshaft 10' is rotated, the valve actuating mechanism 40' will be guided by the guide portion 11' to make one action per two turns of the crankshaft 10'. Thus a valve 44' will be opened and closed at the above-mentioned appropriate frequency.
As the four-cycle engine of said gazette requires no cam shaft nor cam gears, it can be made small in size and light in weight. The engine, yet, has the following margins of improvements:
a) The configuration of the guide portion is complicated. It, therefore, is not easy to form the guide portion. It takes much time to fabricate the portion. Thus it has demerits in terms of production process (delivery time) and cost. PA1 b) The guide portion that turns twice before it returns to the starting point on the crank shafts has a width virtually corresponding to two threads in the axial direction (the direction along the center line of the crankshaft). The crankshaft is longer accordingly, and the casing (crankcase) of the engine is greater as well. Further, the guide portion for the intake valve and the guide portion for the exhaust valve are normally provided on the same crankshaft, independently of each other(a total of two sets), as shown in the drawing. Thus the demerits relating to this point can not be neglected. PA1 c) As mentioned above, each guide portion has an extension in the axial direction, the corresponding part of the valve actuating mechanisms to be guided by the guide portion must travel in the axial direction as well. This means that a joint-like movable part is needed at the ends of the valve-actuating mechanism, resulting in increases in the number of parts, weight and cost accordingly. PA1 d) The guide portions on the crankshaft and the outer bearing away from the oil pan can not be lubricated easily. Therefore, an oil pump is needed to make forced lubrication for above-mentioned portions. The oil pump itself has a certain size, weight and cost.
An objective of the present invention is to provide a light-weight, compact and low-cost four-cycle engine through improvements of the above-mentioned problems.
The four-cycle engine according to the present invention has a gear (an external gear or pinion) on a crankshaft. The gear is made to engage with an internal gear having twice the number of the teeth of the former (engagement inside). Cams are formed on the outer surface of the internal gear, and valve actuating mechanisms are connected to the cams to actuate the intake and exhaust valves to open and close. The internal gear is supported in such a way that the internal gear rotates with its pitch circle constantly being in contact with the pitch circle of the gear on the crankshaft, and the internal gear is located in the casing of the engine. The above-mentioned cams are formed at desired portions of the external surface of the internal gear in such a way that various parts of the cams have different radii relative to the center of rotation thereof and the contact faces of the cams against the valve actuating mechanisms form the desired profile curves. Cams, for example, may be formed into grooves like the case of FIG. 4, being concave relative to other portions. Cams may be formed to be convex, protruding from other portions.
The four-cycle engine of the present invention may be arranged in such a way that the space between said gear and said internal gear is hermetically sealed, a member bisecting the space is placed (with the member, the space on the engaging teeth side is separated from the space on the disengaging teeth side), a suction port of lubricating oil is provided in one of the bisected spaces and a discharge port is provided in the other bisected space, at least one lubricating oil route is connected to the suction port, and at least one lubricating oil route is connected to the discharge port.
Additionally, the crankshaft may be supported by bearings provided at two points across the above-mentioned gear on one side (seen in the axial direction of the crankshaft, one side from the connecting rod) of the cylinder of the single cylinder engine (so-called overhang crank type support).